1. Field of the Invention
This invention relates to an apparatus and method for measuring the temperature of a moving surface. More particularly, the invention relates to such an apparatus and method for measuring the temperature of a moving surface of a metal slab, for example, an aluminum ingot during casting.
2. Description of Related Art
During direct chill (DC) casting of aluminum ingots, the temperature of the ingot surface is controlled by spraying a coolant on the solidified surface of the ingot as it descends from the mould (referred to as secondary cooling). Proper control of this cooling is required to prevent a variety of casting problems and ingot defects. Excessive or insufficient cooling may be responsible for ingot defect formation, such as excessive butt curl, resulting in severe bleeding on short sides of rectangular cross-section ingots, cold folding requiring extra scalping before hot rolling, and cracking caused by excessive thermo-mechanical stresses.
This problem is particularly acute during the start-up phase of such a DC casting process, and a variety of techniques are known to modify the cooling effect of the secondary cooling, such as "pulsed water", as disclosed in U.S. Pat. No. 3,441,079, or gas injection, as disclosed in U.S. Pat. No. 4,693,298. However, these techniques generally involve pre-programming to provide predetermined cooling profiles, and do not use any feedback of ingot conditions to modify the cooling effect. Therefore, they are unable to respond to variations in the coolant properties, mould cooling characteristics, etc.
Related problems arise in a variety of fields which therefore could advantageously use temperature measurements on as-cast strip from a continuous caster, temperature measurements on a slab issuing from a hot mill or temperature measurements on the profile produced from an extrusion press. When these processes in particular are associated with quenching, temperatures change dramatically over short distances.
German patent DE 1,941,816 (assigned to VAW) proposes the use of a temperature measurement sensor which contacts an ingot surface as a means to provide direct temperature feedback to control the coolant application. The apparatus and method are also stated to have applicability in the field of extrusion etc. The sensor is attached to a wheel device which allows repeated contacts to be made between the sensor and the surface at different locations along the ingot as the ingot descends. It is disclosed that the device is used to control coolant flow to compensate for heat flux changes and changes in coolant properties. Such a temperature measurement sensor consists of a pair of contacts of differing metals which, on making contact with the ingot surface, generate a thermal electromotive force (emf) which is representative of the surface temperature at that location. However, this means that the measurement technique relies on uniform surface contact at each location since there is no means of verifying measurement validity, particularly when the temperature varies along the cast ingot, for example, at the start of casting. Furthermore, because of the size of the wheel, the device has limitations as to where it can be used for temperature measurement (for example, there is little room close to a mould surface for a wheeled device to operate), and also limitations as to the spacing between adjacent locations at which the temperature may be measured.
There is therefore a need for a reliable means for measuring the surface temperatures of moving slabs, ingots, or strips, etc., particularly at locations where the temperature may undergo large changes, for example, where a coolant is directed at the surface.